opos6uldev: make the LCD work again

Commit 5d7a95f4 ("imx6ul/imx6ull: synchronise device trees with
linux") removed the display timings from the board device tree whereas
they are still needed by the mxsfb driver.
Add the timings back (the correct ones) in the
imx6ul-opos6uldev-u-boot.dtsi file and remove them from the
opos6uldev.env file.

Update the opos6uldev_defconfig file so that the LCD turns on at boot.

Fixes: 5d7a95f4 ("imx6ul/imx6ull: synchronise device trees with linux")
Signed-off-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>